In conventional audio input stages, in particular in the input stages of musical instrument amplifiers, modules are used for impedance transformation and preamplification. If possible, these modules are operated in their linear operating ranges in order to avoid undesirable distortion products in the output signal. Once this linear working range is left, more-or-less clearly audible and measurable nonlinear distortions occur. To avoid these distortions, measures must be taken to keep the audio input signal below the maximum amplitude that leads to distortions. These measures are either associated with a loss of valuable signal-to-noise ratio or other losses in signal quality.
In the case of electric musical instruments, audio input stages are not only used for the neutral processing or amplification of input signals, they are also used to actively modify sound. To accomplish this, it is necessary in part to transfer extreme transients that have a high dynamic range between the instrument attack and the fundamental tone. With musical instrument amplifiers, the way such dynamic peaks are transferred to a great extent governs the sound of the given device. To achieve the proper transfer, a compromise has to be reached relative to the input amplification. The nature of this compromise is that a portion of the dynamic range—the so-called “headroom”—must be reserved for signal peaks and a somewhat poorer signal-to-noise ratio generally has to be accepted in return. The actual sound is then obtained in the subsequent processing stages, which can involve tone control as well as dynamic compression and distortion. One problem with this approach is that distortion occurs at locations in the circuit that usually have an extremely undesirable effect on the desired result.
Control amplifiers for dynamic compression have been proposed to deal with the problem of dynamic peaks in the processing of audio signals. However, such amplifiers usually have a relatively high level of technical complexity, and they suffer from disadvantages relating to the principle itself—such as audible control processes, slow response, and control intermodulation. The adjustment times required by dynamic compressors, such as, for example, the so-called “attack” and “release”, always require that a relatively lengthy portion of the signal relative to the signal peak also be controlled.